What is the current system?

UK general elections are held under the first past the post, where the voter casts an X for their preferred constituency candidate. The candidate with the most votes wins, regardless of their share of the total number of votes cast.

What is the alternative vote?

AV is designed to secure 50% or more support for MPs by allowing voters to rank the candidates on offer in order (if they want to). So they would put "1" by their first preference candidate, "2" by their second preference, and so on.

If, at the initial count, any candidate receives 50% or more first preference votes, they are declared the winner. But if no candidate gets more than 50%, the candidate with the fewest votes is eliminated and his or her votes reassigned according to the second preferences expressed on the ballot papers. The process goes on until one candidate receives more than 50% of the vote and is declared the winner.

However, it may be that voters chose not to rank all the candidates, and that no candidate gets more than 50%. In that instance, whoever ends up with the most votes wins.

Is AV the same as proportional representation?

No, which is why Nick Clegg, the leader of the Liberal Democrats – long in favour of switching to a PR system – called it "a miserable little compromise" before the general election. Parties can still form a government with less than 50% of first choice votes. The Lib Dems have long supported the single transferable vote, a proportional system that would consolidate existing (and, under AV, single-MP) constituencies into much larger multi-member constituencies.

Do you have to choose more than one candidate under AV?

No. You can vote just for one if you choose to.

Where else is AV used?

AV is used in three countries – Australia, Fiji and Papua New Guinea. It is used in a number of internal elections, such as those for some unions and political parties, for example. Most recently, the Labour leadership election that saw Ed Miliband elected allowed for candidates to be ranked in preference order.

Why AV?

The referendum was the result of coalition negotiations after the general election in 2010, despite the fact that neither the Conservatives nor the Liberal Democrats wanted it.

Tories fear the party would struggle to ever form a majority government again under AV, while the Liberal Democrats campaigned for proportional representation – which AV isn't. The only party for AV was Labour, which promised a referendum in its manifesto but is now the most split on the issue.

When is the referendum?

5 May, the same day as the devolved elections in Scotland, Northern Ireland and Wales, and local elections in Northern Ireland and parts of England.

Many Conservatives are unhappy about the timing, which they say it will lead to a distorted turnout, with those in areas in which there are no elections, such as London, being less likely to vote.

Who is eligible to vote?

Around 46 million people will be eligible to vote, the same as at a general election – British citizens plus Irish and Commonwealth citizens who are resident in the UK. Peers cannot vote at a general election, but can vote in the referendum.

Supporters of a switch to AV say:

• The aim of securing more than 50% of the local vote would ensure MPs work harder to earn and keep voter support. Two-thirds of MPs at the last election were elected on less than a 50% share of the vote.

• It would end the "jobs for life" culture in safe constituency seats (campaigners point to MPs in safe seats who were embroiled in the expenses scandal that hit the previous parliament).

• It would encourage more people to vote, because voters would feel that their say matters more. Campaigners say many are deterred from participating because under first past the post because they feel their vote is wasted.

• It eliminates the need for tactical voting. Electors can vote for their first-choice candidate without fear of wasting their vote.

• A switch to AV would not mean changing the current MP-constituency link.

• Supporters say the system would make it more difficult for extremist parties to win an election, because they would be unlikely to secure many second or third preference votes.

• It encourages candidates to chase second and third preferences, which lessens the attractions of negative campaigning (one doesn't want to alienate the supporters of another candidate whose second preferences one wants) and rewards broad church policies.

Those against AV argue:

• First past the post is the fairest system because it is based on the principle of one person, one vote. AV is a "losers' charter" where the candidate who comes second or third in first preferences can actually be elected.

•Some votes will count more than others: If a voter gives their first preference vote to a mainstream party, their other preferences may not be counted. But if they vote for a fringe party candidate who gets knocked out, their other preferences will count.

• AV is a "politicians' fix" because, instead of the voters choosing the government, it would lead to more hung parliaments and backroom coalition deals.

Critics counter that the current coalition was a result of first past the post and that AV is unlikely to lead to more coalitions because it is not PR.

• AV makes decisive electoral outcomes less likely (critics again point to the 2010 election, which led to an inconclusive win for the Conservatives under first past the post).

• Switching to a new voting system would cost £250m.

• It takes longer to count.

• AV will do little to improve under-represented groups, such as the Greens, in parliament.

The Labour leader, Ed Miliband, is, meanwhile, a high-profile campaigner for the Yes campalongside the Liberal Democrats in the belief that AV would enable "progressive" parties to come together.

Nick Clegg, the party leader who pressed for the referendum, has been urged by Miliband to keep a low profile over fears voters may vote No just to punish him for compromises made in coalition.

The SNP, Plaid Cymru, the Green party, Ukip, the SDLP, Sinn Féin and Northern Ireland's Alliance party support AV (though many would prefer PR), while the DUP and UUP are opposed. Celebrities for and against have also nailed their colours to the mast to broaden the campaigns' appeal.

Why did the Conservatives agree to the referendum if most are opposed?

It was a compromise. David Cameron won a guarantee that the next election will be held under AV only if there is a Yes vote in a referendum and also an equalisation in the size of constituencies – a move that will see the number of MPs fall from 650 to 600 and could benefit the Tories.

What is the exact question being put to voters?

The referendum will say: "At present, the UK uses the "first past the post" system to elect MPs to the House of Commons. Should the 'alternative vote' system be used instead? Yes or no?"

Polls have swung one way and the other. In February, the Yes and No camps were pretty much neck and neck, then a Reuters/Ipsos MORI poll said that, among those certain to vote, 49% supported AV while 37% were against.

The arrow then began to point the other way. A YouGov poll for the Sun showed the No campaign ahead at the end of March, with 44% against AV and only 31% in support. In April, a Populus poll showed a clear lead for first past the post. A Guardian/ICM poll later the same month showed the no campaign 16 points ahead.

As the referendum grows closer, tension is mounting between the two sides, with each accusing the other of dirty tricks. Members of the coalition are also at loggerheads: the energy secretary, Chris Huhne, a Liberal Democrat, has accused his cabinet colleague Lady Warsi of turning to "Goebbels-like propaganda" over claims AV would make mainstream parties pander to the BNP.

What would it mean for the next election if we switched to AV?

No one can reliably predict the electoral impact of AV if it is used in the next election, because much will depend on the relative popularity of the two coalition parties at the time. However, general modelling suggests Labour and the Liberal Democrats would gain seats under AV and the Conservatives would be the losers.

This would go some way to appeasing the Liberal Democrat claim that the number of seats the party gains under first past the post does not reflect their national share of the vote (though this would only be properly redressed under PR).

The real beauty of first past the post is the principle of one person, one vote. Under AV, some votes count more than others.

Why? Because if you vote for a mainstream candidate who is top of the ballot in the first round, your other preferences will never be counted. But if you vote for a fringe party who gets knocked out, your other preferences will be counted.

How can it be right that the second, third, even fourth vote of someone who supports the BNP can count as much as the first vote of someone who supports one of the mainstream parties?

What's more, AV isn't as proportional as people make out. Indeed, if it had been used in 1997, 2001 and 2005, Tony Blair would have got even bigger majorities. Would that really have reflected the will of the country? No. And there's another big unfairness inherent in AV – that candidates who no one really wanted can end up winning.

• Nick Clegg claims AV would help tackle corruption in politics by forcing MPs to listen to voters. Politicians who hold safe seats under first past the post have "jobs for life" and are free to ignore voters, he says.

For years, politicians and parties have courted the votes of a few thousand people in marginal seats and ignored the rest. It is because there are so many MPs with jobs for life that there are so many who can take their constituents for granted.

And it is because there were so many MPs taking their constituents for granted that so many abused their expenses. There was a clear link between how safe an MP's seat was and how likely they were to abuse the system. When a system makes corruption more likely, it should be changed.

The arguments in this referendum have been framed around whether a Yes vote damages David Cameron or a No vote damages Nick Clegg. It is about something more. I want to take, head on, the fear designed to appeal to Labour supporters: that a Yes vote in this referendum will be seen as a vindication of Nick Clegg.

I know this referendum is far harder to win because of Nick Clegg's broken promises. But we can't reduce the second referendum in British political history to a verdict on one man. The change to the alternative vote deserves our support because it is fairer and because it encourages a better politics.

The British people know that the state of our politics is badly broken. Many see Westminster as remote and out of touch.

Politicians should never feel safe or insulated from those they represent. That's what I want to change.